This invention relates to a video occlusion mechanism for overlapping stroke written vectors, and, more particularly, to a video occlusion blanking mechanism using a first full field memory and a second full field memory offset from the first by half a pixel.
Current stroke written displays suffer from overlapping vector distortion causing a number of undesirable effects. On monochromatic displays, for example, an overwriting stroke vector can cause an intensity variation. As another example, on a color display an overlapping stroke vector can cause a color distortion. Two overlapping stroke vectors of different colors can result in a third, undesirable, color being displayed. A mechanism is needed to shut off one of the two intersecting vectors in such cases so that the undesirable effects are eliminated or minimized. The process of dynamically shutting off and then turning on a stroke vector is called occlusion. Occlusion is typically accomplished in a stroke written display by modulating the beam intensity for a period. The beam still traces the stroke vector at the intersection point but its intensity is reduced to a level that does not affect the display.
Prior art systems have utilized a full field memory ("FFM") to accomplish the occlusion function. In a full field memory there is a one-to-one correspondence between each location in memory and an area of contiguous points on the stroke written display. The corresponding area on the display is called a pixel. This one-to-one correspondence is exploited to provide the occlusion function. The FFM is loaded, at the coordinate address of the vector's pixels with an occlusion logical "1" when the vector is drawn. The prior art occlusion works well when an intersecting set of stroke vectors intersect in, or close to a pixel. When the intersection of two stroke vectors occurs at the intersection of four adjacent pixels, however, the full field memory approach of the prior art fails to adequately occlude one of the lines. This problem is called quantization error. Additionally, quantization error results in line width distortion. Full field memory occlusion mechanisms result in occlusion points that are typically smaller in size than the width of the line to be occluded. This results in occlusion that is too small, thus distorting the displayed line. On displays that can present both stroke written information and raster scanned information it may be desirable to occlude the raster signal to the display. Quantization error adversely affects this raster occlusion by not occluding regions at pixel boundaries.
Referring now to FIG. 1A, a schematic diagram explaining the quantization error of two intersecting vectors A and B on a stroke written display with pixels numbered 1, 2, 3 and 4. Vector A is represented by the dotted line and vector B is represented by the line of X's. Vectors A and B intersect at pixel 3. No other pixel shares any part of the intersection. To successfully occlude vector B after vector A has been displayed requires the shutting off of vector B while the display beam is over pixel 3.
In the full field memory approach of the prior art, the x and y coordinates of the vector are used to address the FFM. The FFM provides the occlusion signal directly from memory. In a typical stroke written vector system the vector coordinates are of a higher resolution than the addressability of the FFMs. The most significant bits of the coordinates are used to address the FFMs.
Referring now to FIG. 1B, a schematic diagram of two intersecting vectors C and D on a stroke written display with pixels numbered 1, 2, 3 and 4. Vectors C and D intersect at the common corner point of pixels 1, 2, 3 and 4. Since the vectors have no common intersecting pixel, the FFMs do not contain an occlusion signal at this point. In such cases, the prior art fails to successfully occlude vector D after vector C has been drawn.
Referring now to FIG. 2, a block diagram of a prior art occlusion mechanism utilizing a full field memory having a dimension of 128.times.128.times.1 bits is shown. The full field memory is a static random access memory (SRAM) 10 used to store the video occlusion information in a stroke written display. The SRAM 10 has two addresses, the x address 11 and the y address 12. In this example of the prior art the full field memories are addressed with 7 bits. The output of the full field memory, called the occlusion signal 11, is read directly out of the SRAM 10. The occlusion signal 11 indicates to the occlusion electronics when to modulate the stroke writing intensity to a level that will not affect the display. For systems utilizing a color priority scheme more than 1 bit of information may be employed.
Prior art occlusion mechanisms have not dealt with the overlapping stroke vectors at a pixel boundary problem, the line width distortion problem or the raster scanned video problem. The present invention solves all three problems by employing an additional FFM with an offset address to correct the quantization problem.